Use of a portable time-resolved fluorometer to determine oxytetracycline residue in four fruit crops

Oxytetracycline (OTC) is used worldwide to protect crops against bacterial diseases. The US Environmental Protection Agency approved its use in apple, pear, nectarine, and peach, and set residue tolerance at 350 ng g−1. A europium-sensitised luminescence (ESL) method was developed for in-situ determination of OTC residue in these fruits. After extraction in Na2EDTA-NaCl-McIlvaine buffer at pH 4 and filtration, cleanup was performed using hydrophilic-lipophilic balance cartridges. ESL was measured using a portable time-resolved fluorometer. The signal responded linearly over three orders of magnitude (10–10000 ng g−1) with 17–50 ng g−1 limits of quantitation and 2% averaged relative standard deviation. Recoveries were 84% and 82% at 100 and 350 ng g−1, respectively. Inter-laboratory validation was performed by HPLC–MS/MS.